1. Field of the Invention
The present invention relates to a fixing device which is to be disposed in an image forming apparatus such as an electrophotographic printer or a copier.
2. Description of the Related Art
An image forming apparatus such as a printer or a copier using the electrophotographic process is provided with a fixing device which fixes an unfixed toner image held on a recording member such as a recording sheet or an OHP sheet. As the fixing system, conventionally, a so-called heating roller system is widely used. A fixing device of the heating roller system includes a heating roller having a heat source, and a pressure roller which is pressingly contacted with the heating roller. A recording member holding an unfixed toner image is passed through a nip portion where these rollers are contacted with each other, so that heat and pressure are applied to the recording member. As a result, the toner image is fixed to the recording member.
In such an electrophotographic image forming apparatus, particularly in an image forming apparatus which produces a full color image, a large amount of toner adheres to a recording member. Therefore, it is preferable to configure a fixing device of such an image forming apparatus, so that the length (nip width) of the nip portion in the transport direction is made as large as possible and an unfixed toner image formed on a recording member is fixed to the recording member by heating the toner at a temperature which is as low as possible, and for a long time period.
In the heating roller system of the prior art, however, the nip width depends on the outer diameter of a roller and the thickness of a heat-resistant elastic layer which is formed in the surface layer of the roller. In order to ensure a large nip width, therefore, it is required to increase the outer diameter of a heating roller or to thicken the surface layer. As a result, when a large nip width is ensured in a fixing device for a full color image and of the heating roller system, there arise problems in that the size of the device is increased, and that the reduced heat conduction causes the printing speed to be lowered.
As a countermeasure against the above, a fixing device of a so-called belt system has been proposed which includes a heating belt having a heat-resistant elastic surface layer, and in which the heating belt is pressingly contacted with a pressure roller, thereby forming a nip portion. In a fixing device of the belt system, a large nip width can be easily ensured. As compared with a device of the heating roller system, therefore, a fixing device of the belt system has advantages that the size of the device can be reduced, and that the printing speed can be increased.
A fixing device of the pressuring and heating fixing system, such as the above-described prior art fixing devices of the heating roller system or the belt system has the configuration in which, among the faces of a recording member, the face holding a toner image is directly contacted with the surface of a rotation member such as the heating roller or the heating belt. Therefore, part of an unfixed toner image on the recording member is easily transferred to the rotation member. As a result, such a firing device has a disadvantage that a so-called offset phenomenon in which toner fused to the rotation member is reversely transferred to the rear end of the recording member to soil it, or to a next-transferred recording member to soil it easily occurs. In order to prevent the offset phenomenon from occuring, therefore, a fixing device of the pressuring and heating fixing system is provided with a release agent applying mechanism which applies a release agent to a rotation member so as to improve the release properties between the rotation member and toner.
In the case of a fixing device of the heating roller system, the axial length of the contact face of the release agent applying mechanism which is to be contacted with a heating roller exerts only an effect on the maximum image formation width in the view point that the release agent is to be ensured with respect to a toner image. Even when the axial length of the contact face is somewhat prolonged, or even when a large amount of the release agent is applied, there arises no serious technical problem. In other words, in a fixing device of the heating roller system, a recording member is transported by a heating roller itself, and hence a problem such as a failure of transportation of a recording member is not produced unless a release agent is supplied in a very excessive manner. Furthermore, a functional problem due to the movement of the release agent to the inner side via an end face of the heating roller does not occur. There may arise a case where the release agent is moved to an end face of the heating roller and then along the core. It is considered that such a case can be solved by another technique.
By contrast, the following problems are produced in the above-described prior art fixing device of the belt system. Namely, the release agent which is applied to the outer face of the heating belt by the release agent applying mechanism is moved to the inner face of the heating belt, thereby causing a problem in that slippage occurs between the heating belt and a roller around which the heating belt is wound. This slippage impedes the stable transportation of a recording member and lowers the quality of a fixed image. In a fixing device for a full color image, particularly, a release agent must be applied in an amount which is larger than that required in a fixing device for a monochromatic image, because toners of plural colors are superimposed to one another and a large amount of toner adheres to a recording member. Therefore, the above-mentioned problem becomes remarkable in such a fixing device.
In a fixing device of the belt system, the belt may slightly meander in the width direction (the axial direction of the roller). In such a case, when a release agent applying roller of the release agent applying mechanism is pressingly contacted with the heating belt, there arises another problem in that the inner face of an end portion in the width direction of the heating belt is damaged by the outer face of an end portion of a roller around which the heating belt is wound.